Surface Temperature Changes

Global surface temperature records indicate that the Earth has warmed substantially over the past century (Figure 6.12). For example, the first decade of the 21st century (2000-2009) was 1.4°F (0.77°C) warmer than the first decade of the 20th century (19001909). This warming has not been uniform but rather is superimposed on substantial year-to-year and decadal-scale variability (see Box 6.1), with the most pronounced warming occurring during the last 30 years. Several hypotheses have been put for-

Annual Mean 5—year Running Mean

FIGURE 6.12 Global surface temperature (near-surface air temperature over land and sea surface temperatures over ocean areas) change for 1880-2009, reported as anomalies relative to a reference period of 1951-1980, as estimated by NASA GISS (estimates produced by other research teams are very similar). The black curve shows annual average temperatures, the red curve shows a 5-year running average, and the green bars indicate the estimated uncertainty in the data during different periods of the record. SOURCES: NASA GISS (2010; Hansen et al., 2006, updated through 2009; data available at http://data.giss. nasa.gov/gistemp/graphs/).

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FIGURE 6.12 Global surface temperature (near-surface air temperature over land and sea surface temperatures over ocean areas) change for 1880-2009, reported as anomalies relative to a reference period of 1951-1980, as estimated by NASA GISS (estimates produced by other research teams are very similar). The black curve shows annual average temperatures, the red curve shows a 5-year running average, and the green bars indicate the estimated uncertainty in the data during different periods of the record. SOURCES: NASA GISS (2010; Hansen et al., 2006, updated through 2009; data available at http://data.giss. nasa.gov/gistemp/graphs/).

ward to explain the substantial decadal-scale variability in the surface temperature record, especially the period of relatively flat temperatures from the early 1940s through the late 1970s. Probably the most widely cited hypothesis, which is supported by some statistical analyses and model simulations, is that increasing levels of sulfate aerosols from fossil fuel combustion introduced a cooling effect that offset much of the positive forcing from GHGs during the "flat" part of the record (e.g., Hegerl et al., 2007). This hypothesis seems to be supported by the more pronounced "flattening" in the Northern Hemisphere, relative to the more steady increase in the Southern Hemisphere (where aerosol levels are generally much lower). However, other recent analyses (e.g., Swanson et al., 2009) suggest that natural variations in ocean circulation might also give rise to some of the decadal-scale variations in the global temperature record.

The observed warming is also unevenly distributed around the planet (Figure 6.13). In general, the largest increases in temperature worldwide have occurred over land areas and over the Arctic, which is consistent with the horizontal pattern of warming expected from a positive climate forcing. In the continental United States, on average temperatures rose by 1.5°F (0.81°C) between the first decade of the 20th century and the first decade of the 21st century, or about the same as the global temperature change over this period. There is also a rich tableau of ongoing regional, seasonal, diurnal, and local temperature changes associated with these large-scale, long-term, annual-mean surface warming trends:

• Recent analyses of temperature trends over the Midwest and northern Great Plains have revealed that winter temperatures in that region have increased by 7°F (4°C) over the past 30 years (USGCRP, 2009a).

• Late spring and early summer daytime maximum temperatures in the southeastern United States, on the other hand, declined slightly from the 1950s to the mid-1990s (Portmann et al., 2009).

• An analysis of daily temperature records reveals that during the last decade nearly twice as many extreme record high temperatures have been recorded globally than extreme record low temperatures (Meehl et al., 2009c).

• Hot days and nights have become warmer and more common, while cold days and nights have become warmer and fewer in number (IPCC, 2007a).

Many of these changes are consistent with the spatial and temporal patterns of temperature change expected to result from increasing GHG concentrations.

BOX 6.1

Short-Term Variability Versus Long-Term Trends

When conducting scientific analyses, it is important to analyze data in a manner that is consistent with the phenomenon being studied. Climate, for example, is typically defined based on 30-year averages (Burroughs, 2003; Guttman, 1989). This averaging period is chosen, in part, to minimize the influence of natural variability on shorter time scales and facilitate the analysis of long-term trends, especially trends associated with long-term changes in the Earth's radiative balance. Individual years, or even individual decades, can deviate from the long-term trend due to natural climate variability. Thus, it is not appropriate to look at only a short period of the overall record (such as changes over just the last 5 or 10 years) to infer major changes in the trajectory of global warming.

An example of a more familiar temperature trend—one associated with the seasonal cycle—illustrates the importance of analyzing trends over appropriate time scales.The figure below shows daily average temperatures for New York City for the period of January 1 through July 1, 2009. Temperatures would obviously be expected to increase on average over this 6-month period due to the seasonal cycle, but natural variability (which in this case is largely due to the passage of individual weather systems) also gives rise to significant daily, weekly, and even monthly fluctuations in these data. For example, on February 12, 2009, temperatures reached 51°F and then generally declined to 20°F on March 3 (red arrows). Similarly, temperatures reached 78°F on two days in late April before generally declining to 61°F in mid-June (green arrows). It would be incorrect to conclude that summer was not coming based on these two subsets of the data.

In a similar manner, one could potentially draw erroneous conclusions about the long-term trend in global surface temperature by focusing exclusively on a subset of the data in the figure—such as data from just the last 10 or 12 years (see also Easterling and Wehner, 2009; Fawcett, 2007; Knight et al., 2009). As discussed in the text, the climate system exhibits substantial year-to-year and even decade-to-decade variability, while global temperature increases due to rising GHG increases, and other radiative forcing factors all operate on longer time scales. Robust analyses of global climate change thus tend to focus on trends over at least several decades.a Scientists often average climate data over several years or decades, or use more sophisticated statistical methods, to make long-term trends more readily apparent. Statistical methods can also be used to identify other important climate patterns and trends, such as changes in extreme events or shifts in modes of natural variability.

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